[TowerTalk] Shielded shack

[Jim Lux]

On 9/28/13 8:23 PM, Avery Davis wrote:
> Hans,
> There are professional techniques for using Aluminum foil to shield a
> room, and the trick is to treat the seams and penetrations.
> Penetrations should be installed on a conducting plate (AL to match the
> foil shield, but galvanized steel should be OK), and should use filtered
> or shielded wiring.  This website shows AL foil specifically for room
> shielding, and shows depictions of seam treatment.
>
> http://www.ramayes.com/aluminum_foil_emi_rfi_shielding.htm
>

Note that the foil there (2.5 mil) is substantially thicker than 
household aluminum foil (0.5-0.6 mil)
.

that does have effect.. aside from the lower resistivity of the thicker 
foil, you have to consider skin effect.  A shield works by the incident 
field inducing a current creating an opposite field in the conducting 
plane.  If the foil is thinner than, say, 5 skin depths, some of that 
current is on the opposite surface, where it can radiate.

At 1 MHz (lightning frequencies) skin depth in copper is 2.6 mils and 
aluminum is 3.2 mils

Note that for iron, assuming resistivity of 9.6 micro-ohm-cm (vs 1.67 
for copper) and permeability of 100, the skin depth is 0.6 mils.

This is why galvanized steel is a popular shielding material: cheap and 
very effective in thin layers.


> Here are the seaming methods for AL foil (in order of effectiveness:
> 1.  3" overlap
> 2.  No overlap, 6" AL seaming tape
> 3.  3" overlap plus 3" seaming tape
> 4.  4" wide aluminum tape that has a conductive adhesive.
> I have also seen claims of good results using a folded seam.
>
> But for lightning, the best thing you can do is to implement a single
> point ground.  This is a conductive panel to which all wiring and
> cabling goes to before going anywhere else and has surge/lightning
> protection and shield bonding, and which has an excellent grounding
> bond.  LIghtning is much more likely to follow a conductor into the
> shack than penetrate a non-conducting wall or roof.
>
> 73,
> Avery, WB4RTP
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